Similar instruments have similar frequency responses so they overlap and cover each other up - this is called EQ masking. Carving out space gives you more clarity and allows each instrument to be heard better.

Focus on your main left and right instruments in the mix and give them their own pocket of space in the frequency spectrum

Pick a good sounding mid range frequency on the first track and boost it 3-6db. Cut that same frequency on the second track.

Then find a good sounding mid range frequency on the second track and boost it 3-6db. Cut that same frequency on the first track.

TIP: be sure to carve EQ holes on at least the hard left and right tracks. This not only gives your tracks more clarity, but you increase the perceived width of your mix simply because these outer two tracks have more unique tonal qualities - your ear notices the edges of the mix more.